Recent years have seen wide adoption of common rail type fuel injection systems that are equipped with a common rail for accumulating high-pressure fuel supplied under pressure from a high-pressure pump and are constructed to inject the high-pressure fuel in the common rail into the cylinders of an internal combustion engine through corresponding fuel injection valves at electronically controlled injection timing. For realizing good operating characteristics in this type of fuel injection system, it is preferable, for example, to set the common rail pressure relatively low during idling so as to reduce noise and achieve smooth rotation and to set the common rail pressure somewhat high during low-load operation so as to prevent degradation of fuel efficiency. Further, the common rail pressure is preferably set as high as possible during high-load operation so as to reduce occurrence of black smoke and particulates (PM).
Power deficiency, black smoke and other problems therefore arise if the high-pressure fuel in the common rail is merely supplied to the fuel injection valves as it is over the whole operating range. For overcoming these problems, Japanese Patent Application Public Disclosure No. Hei 8-21332 discloses a common rail type fuel injection system in which a booster piston is provided for increasing the pressure of the high-pressure fuel supplied to the common rail and a controller switches between high-pressure injection with the booster piston operative and low-pressure injection corresponding to the inoperative state of the booster piston.
However, since the disclosed system is structured to selectively supply the fuel injection valves with high-pressure fuel from the common rail or pressure-boosted high-pressure fuel from the booster piston by switching control using two solenoid valves, increased cost cannot be avoided because two sets of solenoid valves and associated drive circuits are required. In addition, the two solenoid valves need to be driven in a required synchronous relationship. In view of the scatter in solenoid valve response characteristics and variation in solenoid valve characteristics with temperature change, however, the required switching characteristic is difficult to achieve over the whole range of use temperatures. Use of a complex and expensive control circuit is therefore unavoidable, so that a problem of high cost also arises from this aspect.
One object of the present invention is to provide a fuel injection system capable of overcoming the foregoing problems of the prior art.
Another object of the present invention is to provide a fuel injection system capable of varying the pressure of fuel supplied to fuel injection valves very rapidly using a simple structure.
Another object of the present invention is to provide a fuel injection system enabling size reduction of a control circuit for high-speed switching the pressure of fuel supplied to fuel injection valves.
Another object of the present invention is to provide a fuel injection system capable of minimizing the level of electrical noise energy output from a driver when the pressure of fuel supplied to fuel injection valves is varied.